"It is quite a bit," Medford police Lt. Mike Budreau said of the number of thefts. "We had 10 for all of December."

Suspects are targeting unlocked mailboxes in residential neighborhoods, mostly in east Medford. Checks and newly-issued credit cards are among the pieces of mail stolen. Suspects are using some information to forge checks at area businesses. Victims are often not aware of the identity theft until several days have passed.

Based on where the financial activity — use of stolen credit cards and forged checks — is occurring, police believe a small group of people, possibly as few as two, are responsible for the crimes.

"We do have some leads in this case for sure," Budreau said. "These people are very active. I think they're working at night. They're specifically looking for flags being up."

Police said residents should not put cash, checks or other valuables to be mailed in an unlocked box. Checks orders should be picked up from the bank instead of having them sent to a home address. Homeowners should also consider getting a locking mailbox.